1. Field of the Invention
The invention relates to a power module.
2. Description of the Related Art
Japanese Patent Application Publication No. 2002-26251 (JP-A-2002-26251) describes one example of a related power module. The power module described in JP-A-2002-26251 has a pair of power devices that are stacked with a plate-shaped output electrode arranged therebetween, and an N-electrode and a P-electrode that are stacked with the pair of power devices therebetween. The power module described in JP-A-2002-26251 is advantageous in that the overall power module is able to be made small because the N-electrode, the first power device, the output electrode, the second power device, and the P-electrode are stacked vertically.
If the output electrode that is positioned between the pair of power devices has a portion that extends in the orthogonal direction (i.e., the direction orthogonal to the direction in which the pair of power devices are stacked) from the area where the pair of power devices are stacked (i.e., the stacked area), that is, if that output electrode has a portion that is exposed and not covered by the pair of power devices, then heat can be released from that exposed portion. With this related power module, the output electrode that is positioned between the pair of power devices is made of material such as copper or copper alloy. The thermal conductivity of these materials is isotropic. That is, the thermal conductivity in the stacking direction and the thermal conductivity in the orthogonal direction are the same.
Forming the output electrode of a material having high thermal conductivity is advantageous for increasing the amount of heat released from the exposed portion (i.e., for increasing the cooling amount by the exposed portion). However, ordinary material has isotropic thermal conductivity, so if the output electrode is formed of material having high thermal conductivity, a large amount of heat will also be transferred between the pair of power devices. As a result, thermal interference tends to occur between the pair of power devices, and the power devices tend to overheat.
On the other hand, forming the output electrode Of material having low thermal conductivity, is advantageous for preventing thermal interference between the pair of power devices. However, as described above, the output electrode has isotropic thermal conductivity, so the amount of heat released from the exposed portion of the output electrode ends up decreasing.